Children's headphones

My DS and DD (9 and 11 years old) have recently started taking the bus to and from school. Like all the other kids, they spend much of the journey listening to music/audiobooks, about 1 to 1.25 hours in total daily. I'm really worried about their ears and damage to their hearing!

Ideally, I'd like to get them headphones that have an appropriate volume limit but from the reading I've done am not totally clear what the right limit is (70dB?). Could anyone give me some advice about how loud is too loud to be safe and / or a recommendation of headphones that might do the trick?

I've just purchased some Sound Sense by Roam headphones for my children for Christmas as we were going to be travelling and they have been amazing, they look really cool but more importantly I feel safe knowing that the built in noise reduction stops them damaging their ear drums, they were also a really great price. I bought them from Amazon if any one is interested but they are also available at roam headphones.com